1. Field of the Invention
The present invention relates to an apparatus for loading a master onto and unloading a master from the rotatable master cylinder of a lithographic printing machine.
In the lithographic printing process, a master of sheet material is first prepared so that it carries a latent image and is then treated so that the imaged areas are ink receptive, that is oleophilic, and the background areas are water receptive and ink repellent, that is hydrophilic. The treated master is applied to a rotating master cylinder or flat bed where moistening fluid and ink are applied to it. A copy sheet may then be pressed against the master to receive the inked image or the image may first be transferred to a blanket roll or plate against which the copy sheet is subsequently pressed to receive the image.
When the direct printing technique is used, the image carried on the master must be reversed to produce a copy which reads correctly. However, when the transfer technique, also called the offset printing technique, is used, the image carried on the master must be right-reading to produce a correctly reading copy.
Lithographic masters may be prepared as described above by any of a number of suitable processes. For example, in the past they have been manually prepared, and loaded and unloaded on the master cylinder by skilled personnel. Accordingly, such processes are relatively difficult and expensive to implement making them uneconomical unless an exceedingly large number of copies of any given master are required. However, it has recently been suggested in U.S. Pat. No. 3,861,306 (DuBois et al.) that lithographic masters be made by an electrostatic process where the master, in the form of a flexible sheet that includes a photoconductive coating, is mounted on an electrically conductive substrate which is uniformly, electrostatically charged. The master sheet is exposed to illumination in accordance with the image indicia on the document to be copied. The illuminated non-imaged master areas become discharged but the non-illustrated, image areas remain charged to form a latent electrostatic image that attracts and retains electroscopic toner particles which may be applied to the master sheet by various techniques.
An etching solution is applied to the master to render the non-image areas hydrophilic and a moistening fluid and lithographic ink are then applied to the master to produce a printable image. Finally, the master is loaded on to the master cylinder and copies are printed therefrom.
The electrostatic master production process may be conveniently automated, thus reducing the need for skilled printing machine operators. Accordingly, this process can be used with particular advantage when relatively small numbers of copies are made from one master, but many masters in a series are required. But to take full advantage of the benefits which result, it is also desirable to load and unload the masters on the master cylinder automatically, and continuously so that relatively short runs of documents can be made economically and efficiently.
The apparatus of the present invention is well suited for use with such lithographic printing machines where the master is electrostatically produced and where it is desirable to load onto and unload the master from a master cylinder in an automatic, continuous, uninterrupted way. However, this invention may be used equally advantageously with lithographic printing machines in which masters are produced by other techniques, but in which it is desirable to automatically load them onto and unload them from the master cylinder.
2. Description of the Prior Art
Automatic master handling apparatus for lithographic printing machines are known. U.S. Pat. No. 3,420,169 (Gammeter) discloses one such apparatus which employs a transfer gripper system that carries a master into the proximity of the master cylinder. A group of master gripper devices are mounted in the cylinder to be extended beyond the cylinder surface at two angularly spaced locations. The transfer gripper system accelerates the master to the peripheral speed of the cylinder and places the master directly in the jaws of the master gripper devices when they are extended beyond the cylinder surface at a first loading position. Subsequently, at a second unloading position rotationally offset with respect to the loading position, the master gripper devices extend beyond the cylinder surface to eject the master. Indeed, as illustrated in the patent drawing, the Gammeter apparatus loads and unloads the master from the cylinder at positions spaced approximately 180.degree. cylinder rotation from each other.
In particular, since masters are loaded and unloaded at nearly diametrically opposed positions of the master cylinder, master handling equipment is mounted on both sides of the printing equipment. Therefore, this arrangement makes it difficult to provide separate modular master handling and printing instrumentalities.
U.S. Pat. No. 3,861,306 (DuBois et al.), noted above, discloses combined electrostatic-lithographic duplicating processes and apparatus. The apparatus described is of modular construction but no detailed disclosure of the loading and unloading apparatus is given.
Other prior lithographic printing machines and master handling apparatus are disclosed in U.S. Pats. Nos. 2,031,136 (Sewick); 2,177,578 (Neidich); 2,220,282 (Ritzerfeld); 2,252,204 (Reilly); 2,352,658 (Richmond et al.); 2,360,015 (Rockhill); 2,398,646 (Karbach et al.); 2,603,154 (Davidson); 2,642,282 (Backhouse); 3,153,380 (Gericke); 3,169,476 (Fielding); 3,190,645 (Reinarkz); 3,221,652 (Mestre); and 3,231,418 (Muggleton). These patents are generally related to such lithographic printing apparatus but are not considered to be as pertinent as the Gammeter and DuBois et al patents.